1. Field of the Invention
The present invention relates generally to idling speed control systems for internal combustion engines, and more particularly, to a system for controlling idling speed of an internal combustion engine, in which extra intake air supplying means is provided for supplying temporarily with extra intake air of a predetermined volume to a combustion chamber of engine when the idling speed goes down to be lower than a predetermined relatively low speed.
2. Description of the Prior Art
There has been proposed an internal combustion engine used for an automobile equipped with an idling speed control system by which a feedback control is carried out for controlling intake air mass flow in an intake passage of the engine to keep idling speed of the engine at a predetermined target idling speed, so as to improve fuel consumption and advancing purification of exhaust gas of the engine. In such an idling speed control system, in order to avoid hunting phenomenon in control which may be caused when the control sensitivity is so increased as to provide the responsiveness unnecessarily quick, a so-called integration control in which a ratio of a variation in the intake air mass flow to a difference between an actual idling speed of the engine and the target idling speed, that is, a control gain is set to be relatively small is generally carried out. However, in the integration control performed with the small control gain, there is a problem that the intake air mass flow can not be sufficiently increased due to the slow responsiveness and therefore the engine is likely to stop operating when the engine is burdened with a relatively heavy load and thereby the idling speed thereof is suddenly lowered.
For the purpose of solving the above mentioned problem inherent in the integration control, there has been also proposed such an idling speed control system as disclosed in, for example, the Japanese patent application publishied before examination under the publication No. 58/53653. In this previously proposed system, when an actual idling speed of an engine has gone down to be lower than a predetermined low speed limit which is set to be lower enough than a target idling speed, intake air of relatively large volume is supplied to a combustion chamber of the engine as extra intake air determined apart from intake air supplied under such a feedback control as mentioned above and thereby the actual idling speed is caused to rise rapidly to exceed the low speed limit. With this repid rising of the actual idling speed, it is optimistically expected that the engine is prevented from allowing the idling speed thereof to be extremely lowered with relatively heavy load and as a result prevented from coming to a stop in its operation.
However, in the previously proposed system, the control for supplying the extra intake air to the combustion chamber of the engine (hereinafter, referred to as extra air supplying control) is commenced when the actual idling speed of the engine has crossed the low speed limit from high to low and then terminated when the actual idling speed has crossed the low speed limit from low to high, and therefore the extra air supplying control once commenced is terminated when the actual idling speed has risen to be slightly higher than the low speed limit, no matter whether the actual idling speed has become high enough to a load imposed on the engine on that occasion or not. Accordingly, for example, in the case where, immediately after the termination of the extra air supplying control, the actual idling speed of the engine has gone down to be lower than the low speed limit again so that another extra air supplying control is required, the lowering of the actual idling speed starts at the speed slightly higher than the low speed limit and progresses rapidly and therefore it is feared that the engine comes to a stop in its operation though another extra air supplying control is commenced.
Further, in the case where the volume of the extra intake air supplied to the combustion chamber of the engine under the extra air supplying control is predetermined to be relatively large in consideration of the above circumstance, there is another problem that the actual idling speed of the engine is caused to make an undesirably sudden rise to be too high under the extra air supply control.